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ElectricalSeverity High: diagnose before driving on

U0121Lost Communication With Anti-Lock Brake System (ABS) Control Module

What it means

A control module stopped receiving messages from the ABS control module on the data bus (CAN). The ABS control module may be without power or ground, or the bus wiring to it is faulty.

Symptoms

  • ABS and ESP warning lights, speedometer may not work
  • Several warning lights at once
  • Other modules may store related codes

Common causes

  • No power or ground at the ABS control module
  • CAN wiring open, shorted or corroded (CAN-H / CAN-L)
  • Low battery voltage during cranking
  • Failed ABS control module

What to check first

  1. Check fuses, supplies and grounds at the ABS control module
  2. Measure CAN-H to CAN-L resistance with the battery disconnected (about 60 Ω on most cars)
  3. Scope CAN-H and CAN-L for a clean mirrored signal
Can I drive with U0121? Not recommended. U0121 can lead to stalling, limp mode or engine damage; diagnose it before driving further.

Generic OBD-II meaning. Manufacturer-specific meanings and checks can differ; always check the service information for the car.

How to test the parts behind U0121

CAN bus networkWiring, typical values, multimeter and scope tests

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U0121: questions and answers

What does U0121 mean?

U0121: Lost Communication With Anti-Lock Brake System (ABS) Control Module. A control module stopped receiving messages from the ABS control module on the data bus (CAN). The ABS control module may be without power or ground, or the bus wiring to it is faulty.

What are the symptoms of U0121?

ABS and ESP warning lights, speedometer may not work. Several warning lights at once. Other modules may store related codes.

What causes U0121?

No power or ground at the ABS control module. CAN wiring open, shorted or corroded (CAN-H / CAN-L). Low battery voltage during cranking. Failed ABS control module.

How do you diagnose U0121?

1. Check fuses, supplies and grounds at the ABS control module 2. Measure CAN-H to CAN-L resistance with the battery disconnected (about 60 Ω on most cars) 3. Scope CAN-H and CAN-L for a clean mirrored signal

Can I drive with U0121?

Not recommended. U0121 can lead to stalling, limp mode or engine damage; diagnose it before driving further.